This invention relates to a low-noise plug valve and more particularly to a plug valve which may be operated in a throttled or partial flow blocking condition at substantially decreased sound levels as compared to conventional plug valves.
The problem of valve noise has been the subject of investigation, and valves have been designed for the purpose of reducing the generated noise. The problem of noise, in a plug-type valve, is difficult to control because of the straight through flow design defined by the plug through passage. Such designs have introduced high noise at medium to high pressure drops, due to flashing and cavitation.
Flashing occurs when the pressure downstream of a control valve, or a restriction, is less than the upstream vapor pressure, so that part of the liquid changes to the vapor state. Accordingly, bubbles of vapor are formed. Downstream, due to pressure recovery, the vapor bubbles rapidly condense back into the original liquid. As this process continues, the bubbles form, grow and collapse downstream, thereby causing conventional cavitation. Not only is this the major source of noise in a valve, it is also a source of physical injury to valve components, by reason of the shock waves which are created by the collapsing of the large cavitation voids or bubbles.
Most noise reduction concepts create a pressure drop in the valve, and the back pressure which is created reduces the possibilities of the liquid dropping below its vapor pressure. Examples of ball valves incorporating flow restrictors in the ball, which rotate with the ball, include Hulsey, U.S. Pat. No. 4,212,321 issued July 15, 1980 and Bey, U.S. Pat. No. 4,530,375 isssued July 23, 1985. In such designs, elements or components are added to the ball, such as by incorporating material in the through port of the ball or in a wall of the ball, providing a tortious or high resistance path for the liquid between the valve inlet and outlet. Such designs are not readily adaptable to plug-type valves, in which a straight or tapered plug, having a through passage, is rotatably mounted in a casing for controlling the flow of liquid from the inlet to the outlet. In particular, such designs do not lend themselves to incorporation into a V-port type of plug valve, in which the flow is roughly proportional to the angular position of the plug within the valve casing.